1. Field of the Invention
The present general inventive concept relates to an image forming apparatus, and more particularly to a sheet feeding unit to automatically feed sheets to a print unit in a body of an image forming apparatus, and an image forming apparatus having the same.
2. Description of the Related Art
A conventional image forming apparatus, such as a laser printer, an ink-jet printer, a multi-function printer, a copying machine, etc., generally include a body defining an appearance of the image forming apparatus, a print unit positioned within the body, a sheet feeding unit to automatically feed sheets to the print unit, and a sheet discharge unit to discharge the printed sheets to an outside of the body. The print unit serves to print black and white images or color images on surfaces of the sheets through application of toner or ink to the sheets supplied from the sheet feeding unit, according to a printing method.
The sheet feeding unit includes a sheet stacking device such as a sheet feeding cassette or a sheet feeding tray on which the sheets are stacked, and a pick-up device to separate and feed the sheets piece by piece from the sheet stacking device to the print unit. The pick-up device includes one or more pick-up rollers which are made of rubber material having a high friction coefficient. Among the sheet stacking devices, the sheet feeding cassette is suitable for an automatic sheet feeding unit for automatic printing due to capability of stacking a plurality of sheets having the same dimensions, whereas the sheet feeding tray is suitable for a manual sheet feeding unit due to capability of permitting a single sheet or a small number of sheets to be quickly stacked thereon, if necessary.
Typically, the sheet feeding cassette is detachably inserted into an installation space defined at a lower portion of the body, and has a sheet lifting plate on which the sheets are placed. The sheet lifting plate has one end supported by a spring, and the other end hingably coupled to one side of the sheet feeding cassette. With the above configuration, the sheet lifting plate raises one end of the sheets stacked thereon toward the pick-up device through a pivot behavior of a predetermined angle about a hinge part. The sheet lifting plate is provided at the one end with a sheet separation mechanism to separate the sheets piece by piece. The sheet separation mechanism is provided to the sheet lifting plate to be brought into contact with one end of the sheets stacked on the sheet feeding cassette so that when the pick-up roller pushes the sheets upward, the sheet separation mechanism blocks the one end of the sheets at the one side of the sheet lifting plate, allowing the sheets to be fed piece by piece without overlapping.
Since such a conventional sheet feeding cassette is configured to allow the spring to elastically support the sheet lifting plate with a constant elastic force, the pick-up device applies a constant feeding force. Thus, the conventional sheet feeding unit has a problem in that, when heavy sheets are stacked on the sheet lifting plate, the sheets can be skewed upon pick-up of the sheets due to an insufficient feeding force.